Spectral gamma ray measurements play a major role in oil & gas logging operations by providing a means for interpreting the porosity-lithology and naturally occurring radioactive material in the downhole environment. This is important to the oil and gas exploration industry because the porosity-lithology of rock formations can be directly correlated to the oil and gas production performance of the certain strata within a field, while the amount of naturally occurring radioactive material is correlatable to specific downhole environmental conditions which are indicative of oil & gas producing formations. Well logging tools use a radioactive source in order to irradiate the surrounding formations and detect returned Compton scattered gamma rays induced from the formation. The gamma rays are sensed downhole by a detector and the number of gamma rays arriving at the detector is inversely proportional to the electron density of the rock, which in turn is proportional to the actual rock density while the energy of the returning gamma rays is a function of the photoelectric capture cross section of the rock, which is indicative of lithology.
Downhole operations are not isothermal, and during logging operations measurement tools experience and must operate over a wide temperature range, typically from surface temperatures to 200 C. In addition, any spectral gamma ray measurement device must operate in a changing thermal environment. The spectral output efficiency of scintillation detectors that have been used in well logging applications to make the spectral gamma ray measurements, however, are dependent on temperature.
Scintillation detectors use any one of a number of phosphorescent materials (usually thallium doped sodium iodide crystal) as a radiation transducer. A gamma ray photon that interacts with the crystal will cause it to emit a number of visible light photons in proportion to the energy of the incident gamma ray. A photomultiplier tube (PMT) converts the visible light into an electrical pulse that is proportional in magnitude to the number of visible light photons that reach its photocathode. The pulse is processed by electronics that accumulate energy and count rate data about the radiation.
The pulse height vs. gamma ray energy measured by a scintillation detector drops as temperature increases because both the light output of the scintillation detector and the gain of the photomultiplier tube (PMT) decrease. The light output of the scintillation detector drops because the crystal material itself becomes less efficient at producing visible light per gamma ray photon, and the optical properties of the detector degrade with temperature. Also, the sodium iodide crystal typically is packaged within a hermetically sealed can, surrounded by reflector material, and optically interfaced to a transparent window. At higher temperatures, the reflector materials may become less efficient, as well as the crystal body, and the interfaces may absorb more visible light photons.
Consequently, such detectors require the use of some gain or energy stabilization approach to adjust system gain to maintain consistency among all radiation measurements over a wide temperature range. Such gain adjustment may be effected by increasing the PMT voltage or amplifier gain.
Current state-of-the-art approaches to gain adjustment generally fall into two energy stabilization control categories: open-loop and closed-loop. Open-loop stabilization systems measure the ambient operating temperature of the crystal and PMT and change the system gain according to a previously measured pulse height vs. temperature system categorization function where certain gain or high voltage power supply settings have been stored in a look-up table. A closed-loop system monitors the pulse height of some isotopic reference, usually an Am-241 Nal(TI) pulser, or some other radiation source, and adjusts the system gain or supply voltage to maintain the peak centroid channel of the reference regardless of temperature.
Open-loop stabilization systems assume that scintillation package light output is a function of temperature only; however, this is not the case due to degradation of the detector over time. Therefore, the compensation system will begin to malfunction after repeated exposure to high temperatures, unless the system is calibrated frequently providing new system gain or high voltage power supply settings. In addition, any degradation of the detector during operation of an open-loop compensation technique will result in inaccurate energy spectral data.
Closed-loop stabilization systems typically use an Am-241 Nal(TI) pulser, or some other radioactive source that is housed within the hermetic package, as a reference. The americium source emits an alpha particle which strikes the Nal(TI) crystal in the pulser package and causes it to emit a greater number of photons than do the gamma rays over the energy range of interest. The system changes gain so the pulser peak remains at a constant centroid location. This system assumes that the pulser light output has a functional relationship with the crystal's light output that is constant in time. This may or may not be true. Also, the assessment of the reference peak centroid location becomes accurate only after several thousand counts accumulate in the peak centroid. However, the calibration sources used generally have a low count rate, so a spectrum must be accumulated for an extended period of time before a correction to the high voltage is made. If the temperature changes within the sampling time, then the gain correction will be inaccurate because the reference peak location will be smeared. In addition, since this gain stabilization approach is predicated on the accuracy and resolution of the isotopic source, the system's measurement dynamic range, accuracy and resolution is dictated by the control signal. The statistical nature of the reference source used in closed-loop systems makes the accuracy of the error signal, and therefore, the correction a function of sampling time. The radioactive source may also introduce unwanted counts in the spectral bandwidth.
Non-radioactive pulsers have also been suggested, such as a pulser driven light emitting diode. In U.S. Pat. No. 4,220,851, a light emitting diode (LED) is driven by an oscillator circuit to emit optical stabilization pulses. The LED is mounted between the scintillation crystal and the photomultiplier tube in an optically transparent disk, so that light flashes of the light emitting diode are also sensed by the photomultiplier tube. As stated in this patent, stabilization and synchronization pulses are generated in the optical portion of the detector and compensation for, and stabilization of variations in gain, both optical and electronic, may be made.